Homeowners are having crucial insurance valuations carried out by companies whose staff lack professional qualifications and are not subject to industry controls.

The Property Institute has expressed concern about the growth of "sum-insured" valuation companies, and Land Information Minister Maurice Williamson has sought advice on the issue.

Since insurance companies began refusing to insure homes on a full-replacement, whatever-the-cost basis, in favour of a cap on how much they will pay out in the event of disaster, numerous private companies have started offering valuation services.

However, some of them do not have registered valuers or quantity surveyors on their staff - and so cannot be challenged if they get their valuations wrong. If valuations are wrong, customers can end up underinsured if disaster strikes, or they may be paying more than they need to for a policy.

Wellington resident Tracy Hinton bought a report from one of the new companies, Sum Insured Associates, at a discounted rate through internet site Groupy.

She paid $387 for the report, which normally costs $774. "They were on site for no more than 30 minutes, they took measurements and asked if they could take photos too," Hinton said.

"[They] left saying that an email report will be sent within the week."

When it arrived, she was surprised by the lack of information in it, as well as inaccuracies. The report said the house had a open fire, which it does not, and did not mention anything about the building materials used, she said.

It estimated the sum insured at $424,000. But Hinton says she does not know if she can trust that figure. "I have no idea if it is right . . . I spotted one inaccuracy, what else is wrong?"

He believed his firm could do a better job than most ordinary people at estimating how much a house should be insured for. "They can do it themselves or, if they want to go to the other extreme, they can get a valuer to do it."

In Hinton's case, he said the house did have an open fireplace, which had been blocked up by a gas heater. The company did not have indemnity insurance to cover mistakes and could not be taken to the valuers' disciplinary board if a property was underinsured as a result of its valuations, he said.

The Property Institute, representing valuers and quantity surveyors, said it was concerned by the growth of unqualified firms. Its chief executive David Clark said there was no regulation that specifically covered them. "There's nothing requiring that someone offering sum-insured valuations be a registered valuer or quantity surveyor."